The Value of the Marginal Kid

Let me expand on a comment I left on one of Bryan's blog posts… I think I'm finally homing in on the argument between Bryan and me about kids. As far as I can tell, Bryan's hypothesis is one of these two propositions:
(a) Given any (non-silly) number of children greater than zero, there IS on average a net benefit to each parent from having one more child.
(b) Given any (non-silly) number of children greater than zero, there WOULD BE on average a net benefit to each parent from having one more child, if they applied the econo-strategies Bryan suggests.
I suspect Bryan's hypothesis is (b). In that case, finding out that (a) is false, as I suspect it is, would be suggestive but not dispositive. But I'm still not sure what evidence would help Bryan actually establish the counterfactual in (b).
It seems like Bryan needs to establish (a) in order to have a strong, Good Morning America-friendly starting point. Something like: “Science says kids are great, more kids are better, and here's how to make more kids better still!” But if he can't establish (a), he'll have to admit that in the normal case, having another kid is negative or neutral for one or both of the parents. So generally there is no selfish reason for the next kid unless you are able to successfully commit to and apply Bryan's clever economist strategies. That just feels a lot less exciting and bookworthy, even if true. But is it?
If ever there was an issue where one ought to expect the effects of Darwinian false consciousness, it would be the value kids. So, if this is supposed to be something like social science, it seems purely anecdotal evidence has to be taken with stiff skepticism. But then what non-anecdotal evidence does Bryan have in support of his counterfactual: that parents would selfishly benefit from the marginal kid were they to apply Bryan's strategies? And if you need to apply the strategies to make the selfish-meter tick upward at all, couldn't you get an even bigger upward tick by applying the same strategies to a smaller number of children?
And then there is the issue of the ability of ordinary people to successfully apply those strategies. How good are most couples at effective Coasean bargaining? (Why aren't they already doing more of it?) Can conservative, Christian middle-American women actually get away with outsourcing a lot more of their childcare without facing social ostracism from their mom-peers? And so on. I remain concerned that Bryan so far has established little more than an argument to the effect that it is possible to make an additional child suck less if youcan manage to apply certain principles. I think this is both indisputable and boring.
Now, it is always open to Bryan to argue in terms of the non-monetary, non-happiness, non-revealed preference value of the next kid. I think we all agree that having kids are meaningful, for example. But I'm not aware of good measures of meaningfulness, and I'd be surprised to find evidence that, say, people with three kids have more meaningful lives than people with two. There is of course always the route of the sentimental moralist, who can appeal to our powerful gut conviction that children (and America and Jesus) are simply WONDERFUL, but that is where even broad-minded economists, like Bryan, rightly fear to tread.
Then again, maybe Bryan does have evidence for (a). But he's already conceded that the evidence isn't there in the happiness data. And there is an obvious downside to an extra kid in terms of lifetime consumption, especially given the income penalty for moms. So what else does he have in mind?